1. Field of the Invention
The present invention relates to a method for manufacturing a solid-state image device.
2. Description of the Related Art
Compared to a related surface irradiation type image sensor, a rear surface irradiation type image sensor has drawn attention which can completely eliminate a decrease in sensitivity caused by reflection (so-called eclipse) at a wiring layer or the like as well as can increase an aperture area. The surface irradiation type image sensor is an image sensor in which a wiring layer is formed closer to a light incident side than a photoelectric conversion region. In addition, the rear surface irradiation type image sensor is an image sensor in which a wiring layer is formed at a side opposite to a light incident side with respect to a photoelectric conversion region.
There has been a plurality of manufacturing methods of image sensors. In the manufacturing methods mentioned above, in general, the formation of a photoelectric conversion region in a complete state is important.
For example, elimination of physical defects, such as scratches and cuts, from the surface of the photoelectric conversion region is important. In addition, elimination of metal contamination from the photoelectric conversion region is also important. Furthermore, it is also important to ensure image pick-up characteristics, such as sensitivity and transfer characteristics. That is, in other words, it is important that the photoelectric conversion region have an ideal profile of various image pick-up characteristics.
In addition, as a particular important point of the rear surface irradiation type image sensor, a hole-accumulation diode (HAD) layer at an incident light surface side is formed uniform. Furthermore, also as a particular important point, the width of the photoelectric conversion region, that is, the thickness of a silicon active layer, is preferably formed uniform.
As one manufacturing method that can satisfy the above important points, a manufacturing method using a silicon on insulator (SOI) substrate has been proposed (for example, see Japanese Unexamined Patent Application Publication No. 2007-13089).
In the manufacturing method in which an SOI substrate is used for forming an image sensor, since the price of the SOI substrate is high, the price of the image sensor is also increased.
In addition, when an SOI structure is formed, a silicon oxide layer, which is called a BOX layer, is inevitably formed. Hence, because of the presence of a silicon oxide layer having a different coefficient of thermal expansion from that of a silicon substrate, the silicon substrate is adversely influenced (for example, generation of warping) in a high-temperature processing treatment, and as a result, the crystallinity and temperature controllability of an active layer are degraded.
As the high-temperature processing treatment, for example, there may be mentioned an activation (annealing) treatment in which ion species introduced primarily by an ion implantation method are activated, or an epitaxial growth treatment which is performed to increase the thickness of an active layer.
For example, in a related furnace type heat treatment which has been frequently used, primarily because of the difference in coefficient of thermal expansion between a silicon substrate and a silicon oxide layer, degradation in crystallinity, such as generation of slip lines, occurs.
On the other hand, in a heat treatment using a lamp heating system (so-called RTA method) which has been widely spread to overcome redistribution of impurities concomitant with the trend toward miniaturization of elements, the substrate is heated by radiation of infrared rays and absorption by a silicon substrate (heat conversion). In a heat treatment by the heating system as described above, instability is induced not only in a heating mechanism but also in a temperature control (radiation thermometer).
In addition, since the silicon oxide layer (BOX layer) is present in the SOI substrate functioning as a barrier for the elimination (such as gettering) of metal contamination in a silicon active layer, the gettering effect is degraded. As described above, the silicon oxide layer considerably restricts the elimination of metal contamination, and hence it becomes difficult to suppress the generation of dark current/white spots.
The gettering is a technique in which the state of absorbing metal contamination species is formed in a region other than an active layer of a silicon substrate, the region having no influence on operation of an element, and in which the degree of cleanness of an element portion is ensured to obtain desired characteristics.
As a general example and as a technique closely relating to the manufacturing method according to an embodiment of the present invention, there has been an intrinsic gettering (IG) method.
In the intrinsic gettering method, in order to form a denuded zone (DZ) layer which is free from defects, a region to be formed into a DZ layer is preferably set apart from that in which an intrinsic gettering layer is formed by slightly more than 10 μm, and the region thus formed is a region free from defects.
In a silicon (active) layer of a current SOI substrate, the state described above is difficult to obtain from a structural point of view, and even if the above state can be obtained, the superiority (such as decrease in parasite capacity) of the SOI substrate is degraded, and hence the use of the SOI substrate is not realistic.
Furthermore, when the gettering function is imparted to a base substrate, the silicon oxide layer (BOX layer) considerably suppress the diffusion of impurities, and hence it becomes difficult to decrease impurities in an active layer region.
On the other hand, in a smartcut substrate, defect generation occurs in a SOI portion due to the generation of a foreign substance in a process, and in a step after the SOI portion is adhered to a support substrate, metal contamination (wiring material) occurs.
First, a smartcut method will be described.
As shown in FIG. 8A, a silicon substrate 111 is prepared.
Next, as shown in FIG. 8B, the surface of the silicon substrate 111 is oxidized to form a silicon oxide layer 112.
Subsequently, as shown in FIG. 8C, hydrogen ions are implanted in the silicon substrate 111 by an ion implantation method to form a split layer 113.
Then, as shown in FIG. 8D, a support substrate 121 is adhered to the silicon substrate 111 with the silicon oxide layer 112 interposed therebetween. For the support substrate 121, for example, a silicon substrate is used. In addition, the support substrate 121 is adhered to a surface of the silicon substrate 111 closer to a side of the split layer 113.
For example, as shown in FIG. 8E, the silicon substrate 111 is peeled away from the split layer 113 so as to leave a silicon layer 114 formed of a part of the silicon substrate 111 which is located closer to a side of the support substrate 121 than the split layer 113.
As a result, as shown in FIG. 8F, an SOI substrate 110 is completed in which the silicon layer 114 is formed on the support substrate 121 with the silicon oxide layer 112 interposed therebetween.
In the smartcut method described above, for example, when a foreign substance 131 is present on the surface of the silicon oxide layer 112 formed on the silicon substrate 111 in an ion implantation step as shown in FIG. 9A, the foreign substance 131 functions as an ion implantation mask. Hence, in the silicon substrate 111 in which a region of the foreign substance is projected, a region in which hydrogen ions are not implanted is formed. As a result, a region 133 in which the split layer 113 is not formed is generated in the silicon substrate 111.
In the state as described above, as shown in FIG. 9B, the base substrate 121 (which is heretofore called the “support substrate 121”) is adhered to the silicon substrate 111 with the silicon oxide layer 112 interposed therebetween, and the silicon substrate 111 (not shown) is peeled away at the split layer 113. As a result, the silicon layer 114 formed of the silicon substrate 111 to be left on the base substrate 121 with the silicon oxide layer 112 interposed therebetween is not left in the region 133 in which the split layer 113 is not formed. That is, the silicon layer 114 (not shown) in the above region is peeled away together with the silicon substrate 111 (not shown) which is peeled away. Furthermore, the silicon oxide layer 112 in the above region is also peeled away. As a result, in the silicon layer 114 on the base substrate 121, a partial void 115 is formed at the portion at which the silicon oxide layer 112 is peeled away together with the silicon substrate 111.
In addition, for example, as shown in FIG. 10A, when the base substrate 121 is adhered to the silicon substrate 111 with the silicon oxide layer 112 interposed therebetween, the foreign substance 131 may enter between the silicon oxide layer 112 and the base substrate 121 in some cases. In this case, the split layer 113 is formed in advance in the silicon substrate 111 by ion implantation.
Subsequently, as shown in FIG. 10B, when the silicon substrate 111 (not shown) is peeled away at the split layer 113, the silicon layer 114 formed of the silicon substrate 111 which is to be left on the base substrate 121 with the silicon oxide layer 112 interposed therebetween is left. In this step, in the region in which the foreign substance 131 (see FIG. 9A) is present, since the adhesion between the silicon oxide layer 112 and the base substrate 121 is weak, the silicon layer 114 (not shown) in the region in which the adhesion is weak is peeled away together with the silicon substrate 111 (not shown) which is peeled away. Furthermore, the silicon oxide layer 112 (not shown) in the above region is also peeled away. Hence, the partial void 115 is generated in the silicon layer 114 on the base substrate 121 at the portion at which the silicon oxide layer 112 is peeled away together with the silicon substrate 111.
Hereinafter, the case in which a solid-state image device (image sensor) is formed using the SOI substrate which is formed as described above will be described. This manufacturing method is a manufacturing method in which the smartcut method is applied to the manufacturing method disclosed in Japanese Unexamined Patent Application Publication No. 2007-13089.
For example, as shown in FIG. 11A, photoelectric conversion portions 141, transistor elements 142, and the like are formed in and/or on the silicon layer 114 of the SOI substrate 101. Furthermore, an insulating layer 143 including a wiring layer (not shown) is formed. In addition, a support substrate 151 is adhered to the surface of the insulating layer 143.
Subsequently, as shown in FIG. 11B, the base substrate 121 (which is indicated by a two-dot chain line) of the SOI substrate 101 is removed by a grinding and a chemical liquid treatment (such as etching by a mixture of hydrogen fluoride/nitric acid). In this case, in a region in which the silicon oxide layer 112 is not formed, etching proceeds. As a result, even the silicon layer 114 is etched and is removed. That is, since the silicon oxide layer 112 is partially removed and is not able to function as an etching stopper, the silicon layer 114 is continuously etched.
Incidentally, in the etching using a mixture of hydrogen fluoride/nitric acid, the etching rate of the base substrate 121 made of silicon (such as single crystal silicon) is approximately 165 times that of the silicon oxide layer 112, and hence the silicon oxide layer 112 functions as an etching stopper.
In addition, as shown in FIG. 12, in an etching proceeding region of the silicon layer 114, wires 145, connection plugs 146, and the like of a wiring layer 144 formed in the insulating layer 143 are corroded by etching species, so that the entire wiring layer 144 is corroded by etching.
Hence, when the smartcut method is used for the manufacturing method disclosed in Japanese Unexamined Patent Application Publication No. 2007-13089, metal species (such as aluminum or copper) of a wiring material may not only cause contamination of production facilities/lines but may also widely cause various types of defects in quality.
Alternatively, after an SOI substrate is formed by the smartcut method, by the use of this SOI substrate, photoelectric conversion portions, transfer gates, and the like are formed in and/or on a silicon layer of the SOI substrate, and a wiring layer is further formed on the silicon layer. Subsequently, after a support substrate is adhered to a side of the wiring layer, a silicon substrate side of the SOI substrate is removed, for example, by grinding, polishing, and/or etching, and further a silicon oxide layer of the SOI substrate is removed by etching. In an image sensor element formed by the manufacturing method as described above, by corrosion caused by the defect of the above silicon oxide layer, only the above image sensor element is a defective.
The phenomenon caused by a foreign substance described above is a fatal problem of the smartcut method, and although the above method may be probably improved, the problem caused by the above phenomenon may not be completely solved nor overcome.